1. Field of the Invention
The present invention generally relates to dynamoelectric machines and stators therefor having cores formed of ferromagnetic laminations. More particularly, this invention relates to a stator core formed to include iron-aluminum alloy laminations in selected locations to reduce electromagnetic (core) losses.
2. Description of the Related Art
Generators, including large turbine-driven generators used in the production of electrical power, generally comprise a rotor coaxially supported within a bore formed by an annular-shaped stator. The rotor serves as a source of magnetic lines of flux that are produced by a wound coil carried on the rotor. The stator comprises a number of conductors in which voltage is induced by the rotor as the rotor rotates within the stator. The stator includes a stator core having slots that contain the conductors, or windings. The slots are located at the inner circumference of the core and extend in the axial direction, defining what may be termed teeth that project radially inward into the bore of the core. In addition to supporting the stator windings, the core must provide a low reluctance path for the lines of magnetic flux.
Stator cores for generators and other dynamoelectric machines are widely formed to have a laminate construction, in which a large number of laminations of a ferromagnetic material are axially stacked. Each lamination includes a layer of electrically insulating material to reduce eddy currents. Laminations are formed to have slots that must be aligned within the stack to define the winding slots of the core. Key bars may be employed at the outer circumference of the core to maintain the alignment of the laminations and the winding slots. Due to cost and material properties, iron-silicon (Fexe2x80x94Si) alloys have been the standard material for stator core laminations, particular for large turbine-driven generators. A typical Fexe2x80x94Si alloy for core laminations is a steel containing about 3 to about 3.5 weight percent silicon, with the balance being iron and low levels of carbon, manganese, etc. Alternative lamination alloys have been proposed, including an iron-aluminum alloy disclosed in U.S. Pat. No. 3,058,857 to Pavlovic et al., and an iron-silicon-aluminum alloy disclosed in U.S. Pat. No. 4,545,827 to Rastogi. The lamination alloy disclosed by Pavlovic et al. is said to contain about 1 to 10 weight percent aluminum, with the balance being essentially iron. Rastogi""s alloy is said to contain, by weight, 0.15 to 0.25 percent silicon, 0.15 to 0.25 percent aluminum, with the balance being iron, controlled levels of manganese, and incidental impurities. Industry practice is to use a single alloy for all laminations of a stator core.
Radial and peripheral magnetic flux components in the core and the axial magnetic flux component (normal to the plane of the stator core-end) induce eddy currents and cause electromagnetic (core) losses in the core laminations. The axial magnetic flux component causes heating and additional eddy current losses in the core-ends, which limit machine capability. Radial and peripheral flux components have been pushed to the limit by magnetic saturation, losses, and mechanical design considerations. Various approaches have been proposed to reduce losses caused by the axial flux component. One such approach is to increase the air-gap length between the stator and the rotor at the core-ends of the stator core. For example, core-end stepping is a commonly-used technique in which the laminations at the core-ends have increasingly larger inner diameters toward the end of the core. While effective, aggressive core-end stepping can reduce the clamping pressure on the teeth of the core. Reducing the relative axial length of the rotor with respect to the stator is another technique that has been employed to reduce the core-end fringing flux. However, the shorter length of the rotor increases the excitation requirement, contributing to lower efficiencies and potentially a bigger thermal challenge.
In view of the above, it would be desirable to reduce core losses and heating, especially in machine core-ends, without sacrificing reliability, efficiency, and performance of a dynamoelectric machine.
The present invention provides an electrical machine stator core and a method for reducing electromagnetic losses and the consequent heating of the core. The stator core makes use of laminations made from an iron-aluminum alloy in place of the standard iron-silicon (Fexe2x80x94Si) at the core-ends.
The stator core of this invention generally comprises a plurality of axially-aligned laminations, each of the laminations having an annular shape defining an outer circumference and an inner circumference. The stator core has oppositely-disposed core-ends and an intermediate portion therebetween. According to the invention, the laminations defining the core-ends are formed of an iron-aluminum alloy, and at least one of the laminations defining the intermediate portion is formed of a ferromagnetic alloy different from the iron-aluminum alloy. According to the invention, placement of the iron-aluminum alloy laminations at the core-ends offers the capability of reducing the electromagnetic losses and consequent heating of the cores, and provides additional potential benefits including improved thermal, electrical, magnetic and mechanical properties, the latter of which includes better ductility for improved machine reliability.
Use of iron-aluminum alloy laminations at the core-ends makes possible a relatively simple technique for generator uprate, by which a partial re-stacking of the core-ends can be an effective solution to reduce core-end losses and temperatures. Retrofitting a stator core in this manner generally comprises removing the laminations located at one or both of the core-ends of the stator core, and then installing replacement iron-aluminum alloy laminations at the core-end, with the result that the laminations located at the core-end are formed of an alloy that is different from the alloy of the laminations remaining at the intermediate portion of the stator core.
A significant advantage of this invention is the ability to reduce core losses and heating of a dynamoelectric machine, especially at the machine core-ends, without sacrificing reliability, efficiency, and performance. A particularly notable advantage is the ability to uprate an existing dynamoelectric machine by partially re-stacking the core-end laminations without necessitating the removal and replacement of all laminations of the stator core. The present invention also makes possible the optimization of a new machine by the selective installation of iron-aluminum alloy laminations at the core-ends.
Other objects and advantages of this invention will be better appreciated from the following detailed description.